The subject matter of the present invention relates to through tubing perforating guns, and more particularly, to a through tubing perforating gun which includes a plurality of phased capsule charges mounted on a single, rigid unitary structure, via a respective plurality of support rings, the structure having an angular cross section and being comprised of a material which will not shatter when the phased capsule charges detonate, the support rings being designed to shatter when the capsule charges detonate.
Perforating guns, used in oil well boreholes for perforating a formation, are comprised of charges mounted on a base structure. The charges are often phased, that is, pointed in different directions, for perforating along a 360 degree circumference of the borehole. Of course, the charges are often not phased. In either case, the charges are mounted on a base strip. The charges may be capsule charges, that is, sealed against ambient pressure, or they may be normal, non-capsule charges, that is, not sealed against such ambient pressure. In the non-phased capsule charge situation, the charges perforate the formation in one direction only. In the phased capsule charge situation, the base strip is often a set of wires or a plurality of tubes connected together by a corresponding plurality of cotter pins. When the phased capsule charges are moving downhole, the wires or cotter pins often break when the charges hit an obstruction in the tubing. In addition, some phased charges are mounted on a straight bar, and a detonating cord is run longitudinally through or around the bar for connection to the charges. While the bar may be rigid enough to withstand an impact with the obstruction in the tubing, the straight bar configuration requires an increased diameter perforating gun relative to other such non-phased capsule charge perforating guns. Therefore, the straight bar, phased charge prior art gun cannot fit within the same tubing string that is used with respect to the non-phased charge perforating gun. One prior art perforating gun, similar to the straight bar, phased charge perforating gun discussed above, is found in U.S. Pat. No. 4,543,703 to Wetzel et al. In this patent, a base carrier, cross sectionally shaped in the form of a polygon, has a plurality of shape charges affixed thereto, a subset of the plurality of charges being fixed to each of the sides of the polygon shaped carrier. The perforating gun of the Wetzel patent appears to be very similar in configuration to the straight bar, phased charge perforating gun (illustrated in FIG. 3 of the drawings). Wetzel suffers from the same disadvantage that is possessed by the straight bar, phased charge perforating gun of FIG. 3; that is, the diameter of the Wetzel gun is increased relative to the diameter of the non-phased charge perforating gun (shown in FIG. 4). Consequently, the gun of the Wetzel patent cannot fit into the same tubing string that is used with respect to the non-phased charge perforating gun. Furthermore, it is possible to utilize non-capsule charges (charges not sealed against adverse ambient pressures) in perforating guns and phase the non-capsule charges; however, when using the non-capsule charges, a carrier is required to surround and protect the non-capsule charges from the hostile ambient fluids, temperatures and pressures often found in a borehole of a oil well.
Therefore, since all the above referenced prior art designs are deficient in some manner, a new perforating gun was designed by applicant whereby capsule charges are used thereby providing good penetration of the formation and eliminating the need for carriers, and such capsule charges are phased without requiring an increase in the diameter of the perforating gun in which the charges are mounted relative to other non-phased charge perforating guns; the new perforating gun is fully disclosed in prior application Ser. No. 07/394,782, filed Aug. 16, 1989, and now U.S. Pat. No. 4,951,744, entitled "Angularly Shaped Unitary Structured Base Strip Comprised of a Specific Material adapted for Phasing Charges in a Perforating Gun", the disclosure of which is incorporated by reference into this specification. In the new perforating gun of the prior application, the capsule charges must be mounted on a base strip which is rigid enough to avoid shattering or severe deformation when the structure impacts an obstruction in a tubing, will allow for phasing of capsule charges mounted thereon, and yet will shatter when the charges in the gun detonate. The new perforating gun is rugged, that is, one which will not become stuck or will not shatter in a tubing when an obstruction is impacted, one which has good penetration of the formation due to its use of capsule (sealed) shape charges, one which phases its charges along at least two directions, and one which does not require an increased diameter tubing string.
However, the base strip of the prior application, on which the charges are mounted, is designed to shatter when the charges detonate; therefore, one cannot determine which charges detonated and which charges did not detonate. In some situations, it may be necessary to determine which charges on the base strip detonated, or which ones did not detonate. Therefore, in order to determine which charges detonated, the base strip should not shatter when the charges detonate, although the charges themselves should be allowed to fall to the bottom of the well following detonation. As a result, following detonation, the base strip may be retrieved for subsequent inspection to determine which charges detonated.